Scanning tunneling microscopes (hereinafter referred to as STM) exploit an effect referred to as electron tunneling which occurs across distances of a few nanometers. According to the principles of quantum physics, an electron will pass through an insulator if the thickness of the insulator is limited to a few nanometers. In other words, an electron orbiting in an electron cloud about an atom residing on a first conductor surface can appear to leap across an insulating gap, if sufficiently small, to an electron cloud about a nearby atom residing on the surface of a second conductor. The above-described effect is referred to as tunneling and the electron flow is referred to as the tunneling current.
Tunneling current is used in STMs to map the surface of a sample. Generally, a probe consisting of a sharp tip or stylus is scanned back and forth over the surface of the sample to be examined. A vacuum, gas or liquid can be used as a tunneling barrier or insulator between the probe tip and sample surface. Topographical variations in the surface of the sample result in changes in the distance separating the probe tip and the sample surface, namely, the distance increases when the probe tip moves across a valley or low spot in the sample surface and decreases when the tip moves over a high point. The variations in distance between the probe tip and sample surface result in proportional variations in tunnel current. Thus, a plot of the tunnel current versus the position of the probe tip yields a topographical trace or picture of the sample surface. A detailed discussion of STMs is provided in "Scanning Tunneling Microscopy" by G. Binnig and H. Rohrer, IBM Journal of Research Development, Vol. 30, No. 4, pp. 355-369, July 1986. See also U.S. Pat. No. 4,343,993 issued to Binnig et al.
In practice, tunnel current changes are used as feedback to reposition the probe tip above the sample surface at a constant height. The probe tip is maintained at a constant height to prevent it from "crashing" into a tall surface protrusion or running over a deep depression or valley in the surface that might result in a loss of tunneling current. The coordinates of the probe tip position within the plane of the sample surface (X and Y coordinates) as well as the coordinate position above the surface (Z coordinate) are then utilized to generate the topographical trace.
As will be readily appreciated from the discussion above, a number of critical factors are involved in STM design in order to achieve accurate results. For example, an incredible degree of mechanical precision is required to "fly" the probe tip at a constant nanometer separation distance over an undulating sample surface. Conventional STMs have employed a combination of coarse and fine positioning mechanisms to control the probe tip position. Generally, the coarse positioning mechanism is a mechanical device having a positioning range on the order of several millimeters that moves the sample into proximity with the probe tip prior to the measurement operation. The actual scanning of the probe tip during the measurement operation is controlled by the fine positioning mechanism.
One method of obtaining fine positioning has been to mount the probe tip on piezoelectric crystals. The crystals change in size when a voltage is applied, thereby permitting small variations in positioning to be accomplished by controlling the voltage applied to the crystals. U.S. Pat. No. 4,894,538 issued to Iwatsuki et al. discloses one example of a piezoelectric type positioning system. The lateral range of piezoelectric positioning systems is usually limited to less than 100 microns.
Another critical factor in obtaining accurate results is the isolation of the probe unit from external sources of vibration. Early laboratory STM designs utilized superconducting levitation to accomplish external vibration isolation, which was not particularly appropriate for practical STM designs. Later generation devices employed spring systems utilizing elongated springs under tensile (stretching) forces or viton dampers, consisting of several metallic stacking plates separated by spacers, to provide vibration isolation. The use of elongated springs to provide vibration isolation, however, leads to a much less compact instrument structure. Viton dampers alone exhibit a higher resonant frequency than spring systems which is also a drawback.
Still another critical factor is expansion and contraction of the STM structure due to temperature variations. Expansion and contraction of the structural elements of the STM can directly impact the critical probe tip to sample surface spacing. At a minimum, the variation in tip to sample spacing due to thermal expansion and contraction will result in error being introduced into the scanning results. In a worst case situation, the probe tip could be damaged by coming into contact with the sample surface due to positioning errors introduced by the thermal expansion and contraction.
The resolution of the STM depends a great deal on the condition of the probe tip. As a result, the probe tip might have to be reworked or replaced if it comes into contact with the sample surface. The probe tip must also be periodically replaced due to normal wear. Replacement of the probe tip in conventional STMs is not particularly convenient. In many cases, the basic structure of the STM must be broken down to permit probe replacement. This is especially disadvantageous when working in vacuum environments, as a good deal of time is lost if the probe tip cannot be readily interchanged without restoring normal atmospheric conditions.
In view of the above, it would be desirable to provide an STM that is compact in structure, is capable of coarse and fine positioning of the probe tip with a high degree of accuracy and reproducibility, provides isolation from external sources of vibration, provides for thermal compensation to counteract thermal expansion and contraction of structural elements, and permits ready replacement of the probe tip.